As an inverter to convert DC power to AC power, a voltage source inverter and a current source inverter are well known. The voltage source inverter performs switching by a solid state switch, between a load and a DC voltage source, thereby supplying a square wave alternating current to the load, functioning as the voltage source.
As one configuration of the inverter, there is known a circuit configuration that an arm of abridge circuit is formed with an inverse-parallel connection between a switching element such as a transistor and a thyristor, and a feedback diode, and PWM control is applied to each switching element, whereby DC-AC conversion is performed.
FIG. 19 is a circuit diagram for describing a general configuration of the single phase inverter circuit. As shown in FIG. 19A, in the single phase inverter, a circuit element formed with the switching element Q1 and the feedback diode D1 being inverse-parallel connected, and a circuit element formed with the switching element Q2 and the feedback diode D2 being inverse-parallel connected, are connected in series, thereby establishing one set of upper and lower arms of the bridge circuit. In addition, a circuit element formed with the switching element Q3 and the feedback diode D3 being inverse-parallel connected, and a circuit element formed with the switching element Q4 and the feedback diode D4 being inverse-parallel connected, are connected in series, thereby establishing another set of upper and lower arms of the bridge circuit. Here, the upper arms of the bridge circuit are connected to a positive terminal, and the lower arms are connected to a negative terminal. A connection point between one set of the upper and lower arms (the switching element Q1 and the feedback diode D1, and the switching element Q2 and the feedback diode D2), and a connection point between another set of the upper and lower arms (the switching element Q3 and the feedback diode D3, and the switching element Q4 and the feedback diode D4) are connected to the load.
The switching elements Q1 and Q4 are driven by a base signal (in the upper part of FIG. 19B), and the switching elements Q2 and Q3 are driven by another base signal (in the lower part of FIG. 19B). Those base signals are in opposite phase with each other, so as to change the direction of the current passing through the bridge circuit, thereby inverting the output voltage (FIG. 19C) and outputting AC current (FIG. 19D). It is to be noted that Q1 to Q4 in FIG. 19D, and D1 to D4 represent devices (switching elements and feedback diodes) through which the output current passes in the bridge circuit. Dead time Td in FIG. 19B is provided to prevent a short-circuit between the upper and the lower arms in the bridge circuit, when the base signal is switched.
There is suggested a configuration that employs soft switching (zero voltage switching (ZVS) and zero current switching (ZCS)) for the switching elements constituting the inverter circuit, so as to reduce switching loss, in performing on and off operations by the switching elements.
A resonance type inverter including a three-phase bridge circuit is known as a soft switching inverter for reducing switching loss. In the resonance type inverter, a commutation diode and a resonant capacitor are connected in parallel with a switching element, and a resonance circuit is configured by the resonant capacitor, a resonant inductor, and the switching element connected to the resonance circuit. Charging and discharging of the resonant capacitor by means of a resonant current of the resonance circuit and conduction of the commutation diode implement zero voltage switching (ZVS) and zero current switching (ZCS) of the switching elements (see the Patent Document 1, for example).
Since the resonance circuit has a configuration that the switching element is connected to the resonant capacitor in parallel, there is a problem that capacitance of the capacitor may increase. In order to solve the problem, there is suggested a configuration where the resonance circuit is formed with an auxiliary circuit including an auxiliary switching element (Patent Document 2).
It is suggested that also in an inverter circuit including a single-phase bridge circuit, an auxiliary circuit is provided in addition to the inverter circuit, so as to perform soft switching (Patent Documents 3 and 4).
In the Patent Document 3, it is described that the first main switch and the second main switch connected in series, are connected in parallel with a diode and a snubber capacitor, the first auxiliary resonance circuit including the first auxiliary switch and the second auxiliary switch being connected in series, and the resonant inductor, is connected to a DC power source, voltage signals of the voltages respectively across the main switches and the auxiliary switches are inputted, and it is controlled such that a turn-on signal as a switching signal is given to the first and the second auxiliary switches, prior to giving the turn-on signal to the first main switch.
In the Patent Document 4, it is described that there is provided an auxiliary circuit for soft switching, including the first to the fourth auxiliary switches, the first to the fourth auxiliary diodes, the first and the second capacitors, and a resonance reactor, and an auxiliary switch control circuit performs on and off control of the auxiliary switches, thereby forming either the first resonance current path including the first auxiliary capacitor, the first auxiliary switch, the resonance reactor, and the fourth auxiliary switch, or the second resonance current path including the second auxiliary capacitor, the second auxiliary switch, the resonance reactor, and the third auxiliary switch.